1. Field of the Invention
The present invention relates to cataphoretic electrodeposition baths and to the use of these baths in cationic electrodeposition of ferrous metal articles, particularly untreated ferrous metal articles.
2. Brief Description of the Prior Art
Electrodeposition of aqueous organic coatings has risen to industrial prominence in recent years. The process has many advantages including uniformity and completeness of coating, even on intricate shapes. The process results in labor savings and pollution abatement. Virtually any electrically conductive substrate can be coated. However, the process has been primarily employed to prime ferrous metal substrates.
Particular interest has recently arisen in cationic electrodeposition coatings, that is, coatings which deposit on the cathode, since when the article is the cathode, there is greatly reduced metal ion dissolution which commonly causes poor corrosion resistance and staining in certain anionic systems.
Examples of suitable cationic electrodepositable resins are those described in U.S. Pat. Nos. 3,799,854 and 3,984,299, both to Jerabek, and 3,947,338 and U.S. Patent Application Ser. No. 648,982, filed Jan. 14, 1976, both to Jerabek, Marchetti and Zwack. The cationic solubilizing groups in these resins are amine acid salts. Also suitable for cationic electrodeposition are quaternary ammonium salt group-containing resins such as those described in U.S. Pat. Nos. 3,962,165 and 4,001,156 to Bosso and Wismer.
Although cationic electrodepositable resins offer significant improvement over anionic electrodepositable resins, it has been noticed that for maximum corrosion resistant coatings over ferrous metal, the metal must be pretreated before it is electrocoated. Examples of pretreatments are the phosphating treatments which include iron phosphating, mixed iron and calcium phosphating and zinc phosphating. Such pretreatments may optionally be followed by a chromic acid rinse. The pretreatment operations, although employed on many industrial electrodeposition lines, are undesirable for a number of reasons. First of all, pretreatments are expensive. Besides the cost of the chemicals themselves, a considerable capital investment is required for equipment. In addition, pretreatments cause pollution problems, the solutions of which add to their expense.
Another and perhaps more important problem associated with pretreatment is the difficulty in applying a uniform pretreatment. With complexed shaped articles, such as automobile rocker panels and the inside surfaces of doors, it is very difficult to apply uniformly the pretreatment chemical. Also, the pretreatment equipment itself may not be operating in a uniform manner resulting in some areas being poorly pretreated. These areas will be the first to corrode and will ruin the entire article. Also, in the case of pretreating steel, there are many grades of steel which may require different degrees of pretreatment. Also, the quality and condition of the steel is important and may not be accurately known by the one doing the pretreatment. Steel of poor quality such as oily steel or steel contaminated with some impurity may detrimentally affect or at least reduce the effect of the pretreatment.
Therefore, it would be desirable to provide a cationic electrodeposition bath which can do away with the need for chemical pretreatment or at least compensate for any shortcomings or deficiencies in the pretreatment process.